Carbon fiber produced through melt-spinning of petroleum pitch or polyacrylonitrile, or vapor grown carbon fiber produced through thermal decomposition of a hydrocarbon compound in an inert atmosphere in the presence of a metal serving as a catalyst is excellent in terms of thermal conductivity, electrical conductivity, mechanical strength, etc. Therefore, a variety of composite materials have been developed, which employs vapor grown carbon fiber in order to, for example, impart electrical conductivity or thermal conductivity to a resin and the like.
Conventionally, a composite material containing carbon fiber with a resin and the like has been prepared by means of a method in which a prepreg is formed from carbon fiber using a binder, or a carbon fiber fabric or the like is formed, and the prepreg or the fabric is impregnated with a resin and the like.
Japanese Patent Laid-Open Publication (kokai) No. 2002-327113 discloses a resin composition serving as a binder for preparing a prepreg of carbon fiber; and Japanese Patent Laid-Open Publication (kokai) No. 2002-180356 discloses a three-dimensional fiber structure formed of carbon fiber, which is to be impregnated with a metallic material.
Meanwhile, preparation of a prepreg of vapor grown carbon fiber is very difficult as compared with the case of a prepreg of the carbon fiber described in the aforementioned patent documents, since, for example, vapor grown carbon fiber has a diameter smaller than that of the aforementioned carbon fiber. Even when a prepreg can be formed from vapor grown carbon fiber, the prepreg contains only small amounts of pores, and therefore, the prepreg fails to be sufficiently impregnated with a material for preparing a composite material.
Therefore, a resin composite material containing vapor grown carbon fiber has generally been prepared by means of, for example, a method in which vapor grown carbon fiber is kneaded with a resin by use of a kneading machine. However, in the method, during the course of kneading, fiber filaments of the carbon fiber are cut into short fiber fragments, and thus the degree of three-dimensional network formation between the fiber filaments becomes insufficient. Therefore, in order to compensate for the thus-cut fiber filaments, a large amount of vapor grown carbon fiber must be added to the resin. However, addition of a large amount of vapor grown carbon fiber causes problems, including deterioration of flow characteristics of the resin during the course of molding, and an increase in production cost.
In an effort to solve such problems, many studies have been made.
Japanese Patent Laid-Open Publication (kokai) No. 2002-348741 discloses vapor grown carbon fiber having a herringbone crystal structure in which end portions of active crystals are exposed to the outside of the carbon fiber. According to this publication, the vapor grown carbon fiber having a herringbone crystal structure exhibits high affinity to a resin and the like; readily assumes an S-like shape, a Z-like shape or a spiral shape when kneaded with a resin; and exhibits good fitting to a resin. Therefore, even when a small amount of the vapor grown carbon fiber is added to a resin, sufficient effects are obtained.
Japanese Kohyo Patent Publication No. 2001-521984 (WO99/23287) discloses a composite material which is obtained through the following procedure: there is prepared a porous material containing vapor grown carbon fiber, each fiber filament of the carbon fiber having a substantially uniform diameter of 3.5 to 70 nm and a substantially cylindrical shape, the porous material corresponding to the aforementioned prepreg of carbon fiber; and the porous material is impregnated with a resin, or polymerization of an organic monomer is performed in the porous material.
Japanese Kohyo Patent Publication No. 8-509788 (WO94/25268) discloses a method for producing a porous material employed in such a composite material, in which vapor grown carbon fiber is dispersed in water or an organic solvent, and then the solvent is removed. Meanwhile, Japanese Kohyo Patent Publication No. 2000-511864 (WO97/43116) discloses that when fiber filaments of vapor grown carbon fiber are melt-bonded or joined together at their contact points by use of an adhesive and/or by means of thermal decomposition, a hard porous material is produced. This patent document also discloses that the hard porous material is produced by means of a method that vapor grown carbon fiber is dispersed in a solution in which an adhesive is dissolved or in a gel fluid, and subsequently the solvent is removed, or the resultant dispersion is subjected to supercritical extraction.
However, the production methods disclosed in the aforementioned publications involve problems attributed to the following: vapor grown carbon fiber is contained in the form of aggregates in the porous material, an operation for applying high shear force is performed in order to separate the aggregates into individual fiber filaments, and excessive kneading is performed by use of a kneading machine during the production of the porous material. When the carbon fiber is contained in the form of aggregates in the porous material or in the resultant composite material, the degree of network formation between fiber filaments of the carbon fiber is lowered as compared with the case where individual fiber filaments of the carbon fiber are present as separated from one -another in the porous material, and thus electrical conductivity or the like of the porous material is lowered, since the electrical conductivity or the like is basically affected by the degree of network formation between the aggregates of the carbon fiber. Therefore, the amount of the vapor grown carbon fiber to be added must be increased, which may lead to lowering of moldability and the like. Meanwhile, when kneading is performed by use of a kneading machine, or fiber filaments of the carbon fiber are dispersed through application of high shear force, the fiber filaments are cut into fiber fragments. Thus the degree of three-dimensional network formation between the fiber filaments becomes insufficient, which causes the problems similar to those aforementioned when the amount of the vapor grown carbon fiber to be added must be increased.
Meanwhile, there have been disclosed a porous molded member formed by mixing and dispersing, in water, a metal or a non-metallic solid, an organic gelable binder such as agar and an inorganic binder, and gelling the resultant dispersion by means of gelation of the organic binder, followed by freeze-drying; and a method for forming a porous molded member characterized by thermally treating the above molded member, thereby curing the inorganic binder (Japanese Kohyo Patent Publication No. 9-505266 (W095/09822)). However, this method involves a problem that the molded member considerably shrinks in volume during the course of thermal treatment.